Hereinafter, radio frame structures, time-frequency resources (radio resources), and HARQ ACK/NACK transmission used in 3rd Generation Partnership Project (3GPP) Release 8 (referred to as Long Term Evolution (LTE)) and subsequent releases will be described. In addition, carrier aggregation (CA) introduced in 3GPP Release 10 (referred to as LTE-Advanced) and HARQ ACK/NACK transmission in CA will be described.
FIG. 1 illustrates a radio frame structure in LTE and LTE-Advanced. In 3rd Generation Partnership Project (3GPP) Release 8 and subsequent releases, two types of radio frame structures are specified. One is referred to as frame structure type 1 and can be applied to frequency division duplex (FDD). The other is referred to as frame structure type 2 and can be applied to time division duplex (TDD). As illustrated in FIG. 1, in both frame structure type 1 and frame structure type 2, the duration of one radio frame is 10 ms, and one radio frame consists of 10 subframes. The duration of one subframe is 1 ms. Furthermore, one subframe is divided into two slots of 0.5 ms each.
FIG. 2A illustrates details of downlink time-frequency resources in LTE and LTE-Advanced. One downlink slot (0.5 ms) includes NDLSYMB orthogonal frequency-division multiplexing (OFDM) symbols in the time domain. A radio resource defined by one OFDM symbol in the time domain and one subcarrier in the frequency domain is referred to as a “resource element.” A resource element is the minimum unit of radio resources in the downlink of LTE and LTE-Advanced using ODFM. A resource unit defined by NDLSYMB consecutive OFDM symbols in the time domain and NRBSC consecutive subcarriers in the frequency domain is referred to as a “resource block.” When a normal cyclic prefix is used, the value of NDLSYMB is 7 and the value of NRBSC is 12 and, therefore, one downlink resource block consists of 84 resource elements. The occupied bandwidth (i.e., NDLRB resource blocks or NDLRB NRBSC subcarriers) depends on the downlink system bandwidth (i.e., channel bandwidth (BWChannel)). For example, if the system bandwidth is 1.4 MHz, the maximum number of downlink resource blocks (NDLRB) is 6, and if the system bandwidth is 20 MHz, the maximum number of downlink resource blocks (NDLRB) is 100.
FIG. 2B illustrates details of uplink time-frequency resources in LTE and LTE-Advanced. One uplink slot (0.5 ms) includes NULSYMB single-carrier frequency-division multiple access (SC-FDMA) symbols in the time domain. SC-FDMA is also referred to as discrete Fourier transform (DFT)-spread OFDM (DFTS-OFDM). Similarly to the downlink, a radio resource defined by one SC-FDMA symbol in the time domain and one subcarrier in the frequency domain is referred to as a “resource element.” A resource element is the minimum unit of the radio resources in the uplink of LTE and LTE-Advanced using SC-FDMA. A resource unit defined by NULSYMB consecutive SC-FDMA symbols in the time domain and NRBSC consecutive subcarriers in the frequency domain is referred to as a “resource block.” Similarly to the downlink, when a normal cyclic prefix is used, the value of NULSYMB is 7 and the value of NRBSC is 12 and, therefore, one uplink resource block consists of 84 resource elements. The occupied bandwidth (i.e., NULRB resource blocks or NULRB NRBSC subcarriers) depends on the uplink system bandwidth (i.e., channel bandwidth (BWChannel)). For example, if the system bandwidth is 1.4 MHz, the maximum number of uplink resource blocks (NDLRB) is 6, and if the system bandwidth is 20 MHz, the maximum number of uplink resource blocks (NULRB) is 100.
In 3GPP Release 8 and subsequent releases, downlink user data is transmitted on a physical downlink shared channel (PDSCH). A wireless terminal (i.e., user equipment (UE)) receives downlink data on a PDSCH, checks whether a cyclic redundancy check (CRC) error is present in the downlink data, and transmits HARQ ACK/NACK bits indicating the result of CRC (i.e., acknowledgement (ACK) or negative ACK (NACK)) on a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH). Specifically, when ULSCH resources are allocated to the UE, HARQ ACK/NACK bits are piggybacked to PDSCH and transmitted. In contrast, when no ULSCH resources are allocated to the UE, HARQ ACK/NACK bits are transmitted on a PUCCH.
In 3GPP Release 8 and Release 9, PUCCH formats 1a and 1b are defined for HARQ ACK/NACK transmission. Basically, the PUCCH format 1a is used to transmit 1-bit HARQ ACK/NACK, and the PUCCH format 1b is used to transmit 2-bit HARQ ACK/NACK. The number of HARQ ACK/NACK bits is determined in accordance with the number of codewords (i.e., 1 or 2) transmitted in the downlink transmission. In other words, the number of HARQ ACK/NACK bits is determined in accordance with whether spatial multiplexing is applied in the downlink transmission. When spatial multiplexing is applied, two codewords (i.e., two transport blocks) are transmitted in a single subframe, and thus the PUCCH format 1b is used.
As illustrated in FIG. 3, the PUCCH is transmitted on the frequency ranges located at both ends of the system bandwidth. The PUCCH transmission in a single subframe is performed by using a single resource block in a first slot (0.5 ms) located at one end of the system bandwidth or in the vicinity thereof and a single resource block in a second slot (0.5 ms) located at the other end of the system bandwidth or in the vicinity thereof. These two resource blocks (RBs) are referred to as an RB pair. As illustrated in FIG. 3, a plurality of RB pairs are used to increase the control signaling capacity. The RB pair (PUCCH region) to be used by the UE can be derived from a PUCCH resource index. The PUCCH resource index is configured in the UE by a base station (i.e., eNodeB (eNB)).
Furthermore, 3GPP Release 10 and subsequent releases define carrier aggregation (CA). In carrier aggregation, a wireless terminal is configured by a base station (eNB) with a plurality of carriers (referred to as component carriers (CCs)) at different frequencies, and can use these component carriers for uplink communication or downlink communication or both. Release 10 specifies carrier aggregation of up to five CCs. The maximum system bandwidth of one CC is 20 MHz, and thus a wireless terminal can use up to 100 MHz in CA of 3GPP Release 10.
To provide a HARQ feedback (ACK/NACK) regarding PDSCH transmission on a plurality of CCs, a new ACK/NACK PUCCH format is defined in Release 10. This new ACK/NACK PUCCH format, which is referred to as a PUCCH format 3, allows HARQ ACK/NACK transmission of up to 20 bits. The PUCCH format 3 is described, for example, in Section 5.4.2A of 3GPP TS 36.211 V12.5.0 (Non Patent Literature 1).
FIG. 4 illustrates a HARQ ACK/NACK transmission scheme using the PUCCH format 3 when a normal cyclic prefix is used. First, in block 401, error-correcting coding and rate matching for the HARQ ACK/NACK bits of a maximum of 20 bits, which have been generated through PDSCH reception, are performed and coded ACK/NACK bits having a length of 48 bits are generated. The coded ACK/NACK bits are scrambled and then mapped to QPSK symbols, and thus 24 QPSK modulation symbols are generated (402). Out of these 24 modulation symbols, 12 modulation symbols (403) are transmitted in the first slot within a subframe, and the remaining 12 modulation symbols (404) are transmitted in the second slot within the subframe. FIG. 4 illustrates a process on the 12 modulation symbols (403) transmitted in the first slot.
Block-wise spreading is performed on these 12 symbols (403). Specifically, the 12 modulation symbols corresponding to one SC-FDMA symbol are spread by the same length-5 spreading code sequence [w(0) w(1) w(2) w(3) w(4)] (404 to 409). This spreading code sequence is also referred to as an orthogonal sequence or an orthogonal cover code (OCC) sequence. The time-domain spreading using the length-5 spreading code sequence provides five sets of 12 modulation symbols. These five sets of 12 modulation symbols are mapped to predetermined five SC-FDMA symbols within the first slot. Each set of 12 modulation symbols is cyclically shifted, DFT-spread, mapped to 12 subcarriers, and converted to a time-domain signal through Inverse Fast Fourier Transform (IFFT), and thus SC-FDMA symbols are generated.